Identification of materials in principal slip zones of faults by X-ray diffraction analysis using a small amount of sample

Abstract

Fault materials in principal slip zones (PSZs), where the shear displacement is concentrated, are one of the most important factors that control the frictional properties of a fault. Thus, the identification of mineral composition of fault materials is essential to understanding the mechanical behaviors of faults. However, since the PSZ of natural faults is commonly very thin (less than a few cm or mm), special attention should be paid to the sampling and analysis of the materials in the PSZ. In this study, we designed X-ray diffraction (XRD) analyses on the fault materials taken from a narrow PSZ using a high-resolution X-ray diffractometer to examine if mineral composition can be reliably determined from a small amount of fault materials. The materials analyzed in the study were taken from about 2-cm-thick PSZ and adjacent fault core of the Yangsan fault zone, Bogyeongsa area, Pohang. Some slabs were prepared from the materials, and the powder specimens used in the XRD analyses were scraped from the slabs with a micro-drill. Given the results of the XRD analyses performed by changing the amount of the material, it follows that the minimum amount of sample needed for the reliable determination of mineral composition is estimated to be about 2 mg. The mineral compositions determined by using the 2-mg-specimens of the PSZ materials (PSZ-B and PSZ-R) and the brownish fault core material (BZ) confirm that the PSZ may be composed of different minerals from the adjacent areas. The method developed in the study may be effectively used for the materials not only in thin layers of natural fault materials but also in recovered experimental faults or in thin faults found in drilling cores from which only a small amount of material can be collected.